Stem cell treatments are a type of cell therapy that introduces new cells into damaged tissue in order to treat a disease or injury. The ability of pluripotent cells to self-renew and differentiate into a range of different cell types offers a large potential to culture tissues that can replace diseased and damaged tissues in the body, without the risk of rejection.
A number of stem cell treatments exist, although most are still experimental and/or costly, with the notable exception of bone marrow transplantation. Medical researchers anticipate one day being able to use cells derived from adult somatic cells to treat cancer, diabetes, neurological disorders such as Parkinson's disease, Huntington's disease, Alzheimer's, dementia, as well as cardiac failure and muscle damage, along with many others.
The reversion of somatic cells to pluripotent cells is commonly referred to as reprogramming. Somatic cell nuclear transfer and cell fusion are examples of techniques employed in the reprogramming of differentiated cells (Lewitzky, M. & Yamanaka, S. (2007) Curr Opin Biotechnol 18, 467-73). Another method of reprogramming was discovered when mouse fibroblasts were reprogrammed with the retroviral introduction of just four transcription factors Oct4, Sox2, Klf4 and c-Myc (Takahashi, K. & Yamanaka, S. (2006) Cell 126, 663-76). Somatic cells can be reprogrammed back to the pluripotent state by the combined introduction of transcription factors such as Oct4, Sox2, Klf4 and c-Myc (OSKM). These converted cells share many characteristics with embryonic stem cells (ESCs) in terms of morphology, genetic expression and epigenetic marks and are known as induced pluripotent stem cells (iPSCs). Since the discovery of iPSCs, cells from different lineages and a diverse range of species have been successfully reprogrammed (Feng, B., et al. (2009) Cell Stem Cell 4, 301-12). There is a need to enhance the efficiency of such methods.
Besides the four reprogramming factors discovered by the groundbreaking study of Yamanaka, other factors such as NANOG and LIN28 were also found to participate in reprogramming (Yu, J. et al. (2007) Science 318, 1917-20). In addition, UTF1, an ESC-specific transcription factor, was shown to enhance the reprogramming of human fibroblasts in conjunction with the four Yamanaka factors as well as the knockdown of p53. Some of the four Yamanaka transcription factors have also been shown to replace factors in reprogramming. For instance, Klf4 can be replaced by Klf2 and Klf5, Sox2 can be substituted by Sox1 and Sox5 while N-myc and L-myc could replace c-Myc. Amongst the four defined reprogramming factors, Oct4 has been shown to be the most critical in inducing pluripotency (Nakagawa, M. et al. (2008) Nat Biotechnol 26, 101-6). However, Oct4 remains irreplaceable by other transcription factors including its close family members such as Oct1 and Oct6 (Nakagawa, M. et al. (2008). No transcription factor has been hitherto shown to be able to substitute Oct4 in the reprogramming of somatic cells.
Oct-4 (an abbreviation of Octamer-4) is a homeodomain transcription factor protein of the POU family. Oct-4 expression must be closely regulated; too much or too little will actually cause differentiation of the cell. Oct-4 has been implicated in tumorigenesis of adult germ cells. Ectopic expression of the factor in adult mice has been found to cause the formation of dysplastic lesions of the skin and intestine. The intestinal dysplasia resulted from an increase in progenitor cell population and the upregulation of β-catenin transcription through the inhibition of cellular differentiation.
Oct4, expressed in the inner cell mass (ICM) of the blastocysts, is critical in maintaining pluripotency of cells in the ICM as well as ESCs. Although neural progenitor cells (NPCs) express a high level of endogenous Sox2, ectopic expression of Oct4 alone was still required for their reprogramming. This observation suggests that Oct4 is pivotal in imparting pluripotency in somatic cells. In addition, only a few transcription factors such as Oct4 and the aforementioned transcriptional factors have been reported to contribute to iPSC generation.
Nuclear receptors have the ability to directly bind to DNA and regulate the expression of adjacent genes. Nuclear receptors are modular in structure and contain specific domains such as DNA binding domain (DBD) and Ligand binding domain (LBD). They are generally classified into two broad classes according to their mechanism of action and subcellular distribution in the absence of ligand. The 48 known human nuclear receptors have been further categorized into subfamilies based on the sequence homology of the proteins. Subfamily 5 includes two nuclear receptors, Nr5a1, also known as steroidogenic factor 1 (Sf1), and Nr5a2. Similar to other nuclear receptors, Nr5a2 possesses a ligand binding domain (LBD) and a DNA binding domain (DBD). However, being an orphan nuclear receptor, the endogenous ligands of Nr5a2 remains unknown. Unlike most nuclear receptors which function as dimers, Nr5a2 is able to bind DNA in its monomeric state (Galarneau, L. et al. (1996) Mol Cell Biol 16, 3853-65).